The present invention relates to a variable speed driving system for an AC electric motor, and more particularly to a large capacity variable speed driving system for driving an AC electric motor using a power converter device provided with a GTO (gate turn-off) thyristor.
A previously proposed variable speed driving system for an AC electric motor with the largest capacity using an inverter is a system with 4000 kVA for general industry such as a pump and a fan. An example of this system is disclosed in e.g. "Toshiba Review" Vol. 43, No. 3 (1983) page 253.
On the other hand, in the field of driving a roller of a rolling mill system, for example, that requires high response with a large capacity, an AC electric motor variable speed driving system based on a cycloconverter is adopted. Its examples are disclosed in Hitachi Hyoron Vol. 72, No. 5 (1990), pp. 441-446; Toshiba Review Vol. 43, No. 3 (1988), p. 256; and Mitsubishi Electric Technical Report Vol. 62, No. 6 (1988), pp. 501-506. An inverter in the field of large-capacity high speed -response such as one used to drive a roller, has not been published.
When conventional systems require a large capacity inverter, a GTO thyristor is used as a switching device in a main circuit of a high- voltage large current device. But, when the device capacity is increased, the permissible switching frequency of the device is reduced (the present limit is 100-150 Hz) so that even when a GTO thyristor is used, harmonics of the output wave from the inverter are increased, the torque ripple of a motor is increased, and loss in the snubber circuit for the GTO is increased (at the rate of square of the voltage).
On the other hand, in the case of the cycloconverter, the only convention device that can be applied to the field of large-capacity high response, the power factor in a power source is limited to around 0.6. In order to improve this power factor, a capacitor with a large capacitance for improving the power factor is required on the power source side. As a result, the resultant apparatus has a large scale.